A prior art DC-DC converter of multi-output type comprises a switching element turned on and off to convert DC input from a DC power source into high frequency power which is supplied to a primary winding of a transformer so that the high frequency power is again converted into a plurality of DC powers through rectifying smoothers connected to a plurality of secondary windings in the transformer to produce a plurality of DC powers from each rectifying smoother. Such DC-DC converters have been used in information appliances such as personal computers and domestic appliances such as air conditioners and audio and visual electric products. For example, as shown in FIG. 35, such a prior art DC-DC converter of multi-output type comprises first and second main MOS-FETs 1 and 2 as first and second main switching elements connected in series to a DC power source 3; a series circuit of a first capacitor 4 for current resonance, a leakage inductance 5d and a primary winding 5a of a transformer 5 connected in parallel to second main MOS-FET 2; a second capacitor 6 for voltage pseudo resonance connected between drain and source terminals of first main MOS-FET 1; a first output rectifying diode 7 whose anode terminal is connected to one end of a first secondary winding 5b of transformer 5; a first output rectifying capacitor 8 connected between a cathode terminal of first output rectifying diode 7 and the other end of first secondary winding 5b; a second output rectifying diode 15 whose anode terminal is connected to one end of second secondary winding 5c of transformer 5; a second output rectifying capacitor 16 connected between a cathode terminal of second output rectifying diode 15 and the other end of second secondary winding 5c; and a stepdown chopper 30 connected to second output rectifying capacitor 16. First output rectifying diode 7 and first output rectifying capacitor 8 constitute a first rectifying smoother 9 to produce a first DC output voltage VO1 from first DC output terminals 10 and 11. Second output rectifying diode 15 and second output smoothing capacitor 16 constitute a second rectifying smoother 17 to produce a second DC output voltage VO2 through stepdown chopper 30 from second DC output terminals 18 and 19.
Transformer 5 has an auxiliary winding 5f electromagnetically connected to primary winding 5a which has a leakage inductance 5d and an excitation inductance 5e. Leakage inductance 5d is equivalently in series to primary winding 5a to allow leakage inductance 5d to serve as a current resonance reactor, and excitation inductance 5e is equivalently in parallel to primary winding 5a. Auxiliary winding 5f is connected to a drive power terminal VCC of a main control circuit 14 as a primary control circuit through an auxiliary rectifying smoother 22 which comprises an auxiliary rectifying diode 20 and an auxiliary smoothing capacitor 21 to supply DC power from auxiliary winding 5f to drive power terminal VCC. A trigger resistor 23 is connected between a positive terminal of DC power source 3 and auxiliary smoothing capacitor 21 to electrically charge auxiliary smoothing capacitor 21 upon start-up of the converter and therefore start main control circuit 14. Connected between trigger resistor 23 and a junction of first and second primary MOS-FETs 1 and 2 is a bootstrap circuit which comprises a rectifying diode 24 and a rectifying capacitor 25 to supply DC power to high side power terminals VB and VS in main control terminal 14. Connected to both ends of first output rectifying capacitor 8 of first rectifying smoother 9 are a first output voltage detector 12 for firstly tracking or discerning first DC output voltage VO1 from first rectifying smoother 9, secondly comparing detected first DC output voltage VO1 with a first regulatory reference voltage, and thirdly producing an error signal VE1, the difference between detected first DC output voltage VO1 and first reference voltage to a light emitter 13a of a photo-coupler 13. Light emitter 13a produces a light which has the irradiative intensity corresponding to an amount of error signal VE1 to forward the light to a light receiver 13b of photo-coupler 13 which transmits error signal VE1 to a feedback input terminal FB of main control circuit 14.
As shown in FIG. 36, main control circuit 14 comprises an oscillator 32 for generating pulse signals VPL of the frequency variable in response to voltage level of error signal VE1 from first output voltage detector 12 to feedback signal input terminal FB through photo-coupler 13; an inverter 33 for producing an inverted signal − VPL of pulse signal VPL from oscillator 32; a first adder 34 for combining a constant dead time with pulse signal VPL from oscillator 32 to produce a first drive signal VG1; a low side buffer amplifier 35 for applying first drive signal VG1 with dead time to a gate terminal of first main MOS-FET 1; a second adder 36 for combining a constant dead time with pulse signal − VPL from inverter 33 to produce a second drive signal VG2; a level shifter 37 for adjusting voltage level of second drive signal VG2 with dead time; and a high side buffer amplifier 38 for applying second drive signal VG2 with dead time from level shifter 37 to a gate terminal of second main MOS-FET 2. As pulse signals VPL are produced with the variable frequency but with the constant pulse duration, main control circuit 14 provides each gate of first and second MOS-FETs 1 and 2 with first and second drive signals VG1 and VG2 while first drive signal VG1 has the on-period of fixed time length and the off-period whose time length is varied in response to voltage level of error signal VE1 from output voltage detector 12 whereas second drive signal VG2 has the off-period of fixed time length and the on-period whose time length is varied in response to voltage level of error signal VE1 from output voltage detector 12. Thus, pulse signals from oscillator 32 serve to alternately turn first and second main MOS-FETs 1 and 2 on and off with the frequency varied in response to voltage level of error signal VE1 from first output voltage detector 12.
As shown in FIG. 35, stepdown chopper 30 comprises a chopping MOS-FET 26 whose drain terminal is connected to a junction between second output rectifying diode 15 and second output smoothing capacitor 16; a flywheel diode 27 connected between a source terminal of chopping MOS-FET 26 and a secondary negative output terminal 19; a filter reactor 28 connected between a junction of source terminal of chopping MOS-FET 26 and cathode terminal of flywheel diode 27 and a secondary positive output terminal 18; and a filter capacitor 29 between positive and negative output terminals 18 and 19. A chopping controller 31 comprises an inner generator (not shown) for producing a second regulatory reference voltage, and produces PWM (pulse width modulation) signals VS2 whose pulse width is modulated relative to an error signal, the difference between a second DC output voltage VO2 across filter capacitor 29 and second regulator reference voltage. Step-down chopper circuit 30 functions to control the on-off operation of chopping MOS-FET 26 by means of PWM signals VS2 from chopping controller 31 to produce from second DC output terminals 18 and 19 second DC output voltage VO2 of constant level lower than DC voltage from second output smoothing capacitor 16 of second rectifying smoother 17.
In operation of prior art current resonance DC-DC converter of multi-output type shown in FIG. 35, a main power switch not shown is turned on to apply power voltage E from DC power source 3 through trigger resistor 23 to auxiliary smoothing capacitor 21 of auxiliary rectifying smoother 22 and thereby charge auxiliary smoothing capacitor 21. When auxiliary smoothing capacitor 21 is charged up to a start-up voltage, main control circuit 14 starts operation so that main control circuit 14 produces first and second drive signals VG1 and VG2 to each gate terminal of first and second MOS-FETs 1 and 2 to commence the on-off operation of first and second main MOS-FET 1 and 2. During the on period of first main MOS-FET 1, electric current IQ1 flows from DC power source 3 through current resonance capacitor 4, leakage inductance 5d and primary winding 5a of transformer 5 and first main MOS-FET 1 to DC power source 3. At the same time, first secondary electric current flows from first secondary winding 5b of transformer 5 through first output rectifying diode 7 to first output smoothing capacitor 8 of first rectifying smoother 9, and under the influence of first secondary electric current, a first load current flows through current resonance capacitor 4, leakage inductance 5d and primary winding 5a of transformer 5 and first main MOS-FET 1. In addition thereto, a second secondary electric current flows from second secondary winding 5c of transformer 5 through second output rectifying diode 15 to second smoothing capacitor 16 of second rectifying smoother 17, and under the influence of second secondary electric current, a second load current flows through current resonance capacitor 4, leakage inductance 5d and primary winding 5a of transformer 5 and first main MOS-FET 1. Moreover, an excitation current flows through current resonance capacitor 4, leakage inductance 5d and excitation inductance 5e of transformer 5 and first main MOS-FET 1. Accordingly, winding current IQ1 flowing through first main MOS-FET 1 is a composite current of first and second load currents and excitation current. First and second load currents are sinusoidal wave-formed resonant currents with the resonance frequency determined by capacitance of current resonance capacitor 4 and leakage inductance 5d of transformer 5. Excitation current is a resonant current with the resonance frequency determined by capacitance of current resonance capacitor 4 and a composite inductance of leakage and excitation inductances 5d and 5e of transformer 5, and resonant current is observed as a triangular wave-formed current which has oblique sides composed essentially of a part of sinusoidal wave because the resonance frequency is lower than switching frequency of first main MOS-FET 1.
When first main MOS-FET 1 is turned off, energy accumulated in transformer 5 by excitation current causes voltage pseudo resonance so that voltages VQ1 and VQ2 between drain and source terminals of first and second main MOS-FETs 1 and 2 become pseudo resonance voltages with the resonance frequency determined by composite inductance of leakage and excitation inductance 5d and 5e of transformer 5 and composite capacitance of current resonance and voltage pseudo resonance capacitors 4 and 6. Specifically, when first main MOS-FET 1 is turned off, electric current IQ1 flowing through first main MOS-FET 1 is diverted to voltage pseudo resonance capacitor 6 so that diverted current electrically charges voltage pseudo resonance capacitor 6 to power voltage E of DC power source 3, and thereafter is commutated toward an inner diode not shown in second main MOS-FET 2. In other words, energy stored in transformer 5 by excitation current causes diverted current to run through inner diode in second main MOS-FET 2 to electrically charge current resonance capacitor 4. Accordingly, second main MOS-FET 2 is turned on during this charging period to accomplish a zero volts switching (ZVS) of second main MOS-FET 2.
When energy stored by excitation current in transformer 5 is completely released, energy stored in current resonance capacitor 4 causes a circulation current to flow from current resonance capacitor 4 through second main MOS-FET 2, excitation and leakage inductances 5e and 5d of transformer 5 to current resonance capacitor 4 to discharge the energy. In other words, excitation current flows in the adverse direction to that during the on-period of first main MOS-FET 1. This excitation current is a resonant current with the resonance frequency determined by a composite inductance of leakage and excitation inductances 5d and 5e of transformer 5 and capacitance of current resonance capacitor 4, and excitation current is observed as a generally triangular shaped current whose oblique sides approximate to a part of sine wave because resonance frequency of excitation current is lower than switching frequency of second main MOS-FET 2.
FIGS. 37(A) and (B) show waveforms of voltage between drain and source terminals of first main MOS-FET 1, electric current IQ1 flowing through drain and source terminals of first main MOS-FET 1 and voltage VC2 applied on current resonance capacitor 4 when input voltage E from DC power source 3 is respectively high and low with the unchanged on-period of first main MOS-FET 1 and the varied on-period of second main MOS-FET 2. In other words, FIGS. 37(A) and (B) demonstrate variation in voltage VC2 across current resonance capacitor 4 under on-duty control of first main MOS-FET 1 by changing the on-period of second main MOS-FET 2 relative to fluctuation in input voltage E. This results in change in voltage applied on primary winding 5a of transformer 5 to control first DC output voltage VO1. FIGS. 37(C) and (D) represent waveforms of voltages VQ1 and VC2 and electric current IQ1 under the respectively light and heavy load conditions. As shown, FIGS. 37(C) and (D) indicate electric current IQ1 flowing through first main MOS-FET 1, and electric current IQ1 of FIG. 37(C) has the generally triangular waveform under the light load condition with less amount of resonant current as a load current, whereas electric current IQ1 of FIG. 37(D) contains sinusoidal variation as part of the waveform resulted from resonance current corresponding to load current under the heavy load condition. In this case, the period for supplying electric power from primary to secondary side of transformer 5 is determined by resonance frequency given by current resonance capacitor 4 and leakage inductance 5d of transformer 5 so that the on-period of first main MOS-FET 1, namely the period for furnishing electric power from primary to secondary side of transformer 5 is almost unchanged if load fluctuates. FIGS. 37(C) and (D) prove unchanged on-periods of first main MOS-FET 1 with a same time length while voltage VQ1 between drain and source terminals of first main MOS-FET 1 is kept on zero volt.
FIG. 38 is a graph indicating variation of first DC output voltage VO1 with change of the on-period ratio or duty ratio of first main MOS-FET 1 to second main MOS-FET 2. As understood from FIG. 38, prior art resonant DC-DC converter of multi-output type shown in FIG. 35 can adjust first DC output voltage VO1 from first DC output terminals 10, 11 by varying the on-period ratio of first main MOS-FET 1 to second main MOS-FET 2 in a range from 0.3 to 1.0. In other words, change in the on-period ratio between first and second main MOS-FETs 1 and 2 causes adjusting charged voltage VC2 in current resonant capacitor 4 to thereby control voltage on primary winding 5a of transformer 5 and also first DC output voltage VO1 produced from first DC output terminals 10 and 11 through first secondary winding 5b of transformer 5 and first rectifying smoother 9.
First output voltage detector 12 picks out first DC output voltage VO1 between first DC output terminals 10 and 11 to produce from first output voltage detector 12 an error signal VE1, the differential between a reference voltage for regulating the first output voltage value and the detected voltage from first output voltage detector 12 so that error signal VE1 is transmitted to a feedback input terminal FB of main control circuit 14 through light emitter and receiver 13a and 13b of photo-coupler 13. Main control circuit 14 produces first and second drive signals VG1 and VG2 whose pulse frequency is modulated depending on voltage level of error signal VE1 applied to feedback input terminal FB from first output voltage detector 12, and supplies them to each gate terminal of first and second main MOS-FET 1 and 2 to alternately turn them on and off with the frequency in response to voltage level of error signal VE1 from first output voltage detector 12. This serves to control first DC output voltage VO1 from first DC output terminals 10 and 11 toward a substantially constant level.
On-off operation of first and second main MOS-FETs 1 and 2 invites on second secondary winding 5c of transformer 5 a voltage which is impressed on second rectifying smoother 17. At this time, according to turn ratio between first and second secondary windings 5b and 5c of transformer 5, produced across second output smoothing capacitor 16 is a DC voltage which is applied to a stepdown chopper 30. A chopper controller 31 in stepdown chopper 30 compares voltage VO2 across a filter capacitor 29 with reference voltage for regulating second output voltage value and produces a PWM (Pulse Width Modulation) signal VS2, the differential between voltage VO2 and reference voltage. Stepdown chopper 30 utilizes PWM signals VS2 from chopper controller 31 to control the on-off operation of a chopper MOS-FET 26 and thereby generate from second DC output terminals 18 and 19 a second DC output voltage VO2 of a constant level lower than DC voltage applied to second output rectifying capacitor 16.
A typical flyback or forward DC-DC converter of multi-output type can change an on-off duty ratio of main switching elements provided in primary side to control DC outputs generated in secondary side while varying a period of time for supplying electric power from primary to secondary side of transformer 5. This gives rise to a drawback in that the on-off duty ratio determined by DC output voltage from one of secondary windings concomitantly restricts electric power drawn from the other of secondary windings, thus resulting in reduction in output voltage produced in the other of second windings. On the contrary, a current resonant DC-DC converter of multi-output type has an important advantage of less change in the period of time for transmitting electric power from primary to secondary side of transformer 5 even under variation of electric load connected to first DC output terminals 10 and 11 because the period of time for transmission of electric power is determined by a resonance frequency depended on current resonant capacitor 4 and leakage inductance 5d of transformer 5 in primary side. In this way, second secondary winding 5c of transformer 5 can produce a necessary amount of electric power therefrom without inducing declination in output voltage from second rectifying smoother 17 whether electric load is light or heavy. However, it has been found that second rectifying smoother 17 actually produces fluctuating output voltage because transformer 5 does not have an ideal electromagnetic coupling of windings and also second rectifying smoother 17 is subject to fluctuation in input voltage E from DC power source 3 and impact by voltage drop in first rectifying smoother 9. To avoid these defects, DC-DC converter shown in FIG. 35 employs stepdown chopper 30 for steadying DC voltage from second rectifying smoother 17 to develop a stable second DC output voltage VO2 from second DC output terminals 18 and 19. Specifically, stepdown chopper 30 provided at a subsequent stage of second rectifying smoother 17 can provide a current resonant DC-DC converter of multi-output type for an ideal cross-regulation which means an output voltage fluctuation for one of electric loads associated with change in the other of electric loads in a prescribed range.
In addition, Patent Document 2 as below discloses a DC-DC converter of multi-output type which comprises a transformer provided with a primary winding and first and second secondary windings for power conversion, a field effect transistor connected to primary winding of transformer for the switching operation, a first voltage detector for detecting stabilized output voltage from first secondary winding of transformer, a first pulse width regulator for comparing detected output from first voltage detector with a reference voltage to control the pulse width of pulse controlled signals to field effect transistor, a switch circuit connected to one end of second secondary winding of transformer, a second voltage detector for detecting rectified and smoothed output voltage from second secondary winding of transformer, a second pulse width regulator for comparing detected output from second voltage detector with a reference voltage to control the pulse width of pulse controlled signals to the switch circuit and a synchronization circuit for synchronizing outputs from second pulse width regulator with outputs from first pulse width regulator. This DC-DC converter controls the on-period of switch circuit in response to output voltage from and connected to second secondary winding of transformer not for feedback to primary side to stabilize the output voltage while reducing power loss under large fluctuation in load of an output system for feedback to primary side.
[Patent Document 1] Japanese Patent Disclosure No. 3-7062 (Page 5, FIG. 1)
[Patent Document 2] Japanese Patent Disclosure No. 2000-217356 (FIG. 2 on page 4 and FIG. 1 on page 5)